Method for evaluating cation-exchange resin and method for controlling water treatment system using the same

ABSTRACT

A method for evaluating a strongly acidic cation-exchange resin wherein said strongly acidic cation-exchange resin is contacted with an aqueous eluting solution and polystyrenesulfonic acid being eluted from this resin is measured, which comprises the steps of setting a plurality of molecular weight ranges in the molecular weight distribution of the polystyrenesulfonic acid eluted, and evaluating the performance of the cation-exchange resin based on the relationship of each molecular weight range with the amount eluted in said each molecular weight range. The evaluation method allows precise evaluation of the performance of a cation-exchange resin independently of the structure of the resin matrix and the circumstance under which it is used, which leads to the determination of an optimum timing for replacement of a resin in a water treatment system using a cation-exchange resin, and thus to the extension of an replacement cycle of the resin and the reduction of an operation cost for the system.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for evaluating acation-exchange resin and a method for controlling a water treatmentsystem using the same, and more specifically, to a method fordetermining a deterioration degree of a strongly acidic cation-exchangeresin used in a condensate demineralizer of a power plant such as afossil-fueled electric power plant and a pressurized water reactor-typenuclear power plant (hereinafter, also referred to as “PWR-type nuclearpower plant”), and a method for controlling a water treatment systemusing the same.

BACKGROUND ART OF THE INVENTION

A strongly acidic cation-exchange resin, which is combined with astrongly basic anion-exchange resin to form a mixed bed and used in acondensate demineralizer of a fossil-fueled electric power plant or aPWR-type nuclear power plant, is usually a cation-exchange resincontaining sulfonic acid groups as exchange groups, and the mainconstituent of substances eluted from the resin is polystyrenesulfonicacid (hereinafter, also referred to as “PSS”). It is known that, in acase where a strongly acidic cation-exchange resin has been degraded byoxygen, PSS is eluted, the eluted PSS fouls the anion-exchange resin,and the demineralization performance of the anion-exchange resindeteriorates. Moreover, when the degradation of the demineralizationperformance progresses, impurity substance ions leak at an exit of thedemineralizer, and a degree of purity necessary for treated water cannotbe ensured.

Therefore, it becomes necessary to adequately evaluate an ongoingperformance of such a cation-exchange resin used in a condensatedemineralizer and always to use a resin which has not degraded to anunacceptable degree. As a common method for evaluating the performanceof a cation-exchange resin is to determine the PSS elution tendency. Inpractice, a method is employed wherein a strongly acidic cation-exchangeresin is dipped or agitated in an aqueous extracting solution and aftera predetermined time an amount of PSS eluted into the aqueous extractingsolution is determined (for example, JP-A-9-210977). In this method, anamount of eluted PSS having a molecular weight of 10,000 or more, forexample, is employed as an index of deterioration.

However, from recent investigation results, it has been recognized that,depending upon the nature of the structure of the matrix of a stronglyacidic cation-exchange resin and the circumstance under which it isused, there is a case where a proper evaluation cannot be achieved bythe conventional method for merely determining the amount of PSSelution. For even if the same category of strongly acidiccation-exchange resins are used, for example, there is a difference indistribution of molecular weight of eluted PSS between a gel-typestrongly acidic cation-exchange resin and a porous-type strongly acidiccation-exchange resin, and when the distribution of molecular weight ofeluted PSS is different between these resins, the degree of influenceupon the PSS reaction to a strongly basic anion-exchange resin variestoo.

DISCLOSURE OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodcapable of more precisely achieving evaluation of a performance of acation-exchange resin independently of the structure of the resin matrixand the circumstance under which the resin is used, and a method forcontrolling a water treatment system using the same.

To accomplish the above object, a method for evaluating acation-exchange resin according to the present invention wherein thestrongly acidic cation-exchange resin is contacted with an aqueouseluting solution and polystyrenesulfonic acid being eluted from theresin is measured, comprises the steps of setting a plurality ofmolecular weight ranges in a molecular weight distribution of thepolystyrenesulfonic acid eluted; and evaluating a performance of thestrongly acidic cation-exchange resin based on a relationship of eachmolecular weight range with an amount eluted in said each molecularweight range. Namely, merely the total amount of PSS eluted is not paidattention to, but this method is a new evaluation method for payingattention to an elution amount in each molecular weight range.

Especially, in this method, it is preferred that a weighting factor forindicating a degree concerning the performance of the strongly acidiccation-exchange resin is preset for said each molecular weight range,and the performance of the strongly acidic cation-exchange resin isevaluated by using the sum of values, each calculated by multiplying anamount of PSS eluted in said each molecular weight range by acorresponding weighting factor, as an index indicating the performanceof the strongly acidic cation-exchange resin. By using such a sum as theindex, it becomes possible to determine the resin performance only bythis sum.

The above-described weighting factor can be set by various methods. Forexample, the weighting factor can be preset for said each molecularweight range, based on a variation degree of a property of a stronglybasic anion-exchange resin ascribed to PSS eluted from the stronglyacidic cation-exchange resin when this resin is used in a form of amixed bed with an anion-exchange resin. More specifically, for example,the weighting factor for said each molecular weight range can be setbased on a variation degree of a property of an anion-exchange resinexhibited when a representative molecular weight (a median value or so)is set for said each molecular weight range and a standard PSS havingthe representative molecular weight as a known molecular weight isadsorbed on the anion-exchange resin. In this case, as the property ofthe anion-exchange resin for observing the variation degree, a masstransfer coefficient (hereinafter, also referred to as “MTC”) of theanion-exchange resin can be employed, and further, the weighting factorcan also be set based on a variation degree of a demineralizationperformance of the anion-exchange resin.

As the above-described molecular weight range of PSS, it is preferredthat a plurality of molecular weight ranges are set in a broader rangeof molecular weight of 10,000 or more. Because a PSS having a molecularweight less than 10,000 does not contribute little if any to thedegradation of the performance of the strongly basic anion-exchangeresin, it is possible to eliminate it from the determination factor inthe evaluation of the resin performance and even if eliminated, theaccuracy of the evaluation of the resin performance according to thepresent invention is not substantially influenced.

Further, in the method for evaluating a cation-exchange resin accordingto the present invention, it is possible to evaluate a resin sample bydeteriorating the resin sample acceleratedly. For example, it ispossible that copper and/or iron ions are adsorbed on the stronglyacidic cation-exchange resin, a hydrazine aqueous solution is contactedwith the resin to deteriorate it acceleratedly, and after the copperions and/or the iron ions are desorbed, the hydrazine aqueous elutingsolution is contacted to elute PSS acid into the aqueous elutingsolution, and the resin sample is evaluated by the above-describedmethod. In this case, for example, an aqueous solution containingammonia and hydrazine can be used instead of the hydrazine aqueouseluting solution.

The above-described method for evaluating a cation-exchange resinaccording to the present invention is suitable, in particular, for usein evaluating the performance of a cation-exchange resin used in acondensate demineralizer of a fossile-fueled power plant or a PWR-typenuclear power plant.

In such a method for evaluating a cation-exchange resin according to thepresent invention, as is evident from the examples described later,independently of the structure of a resin matrix and the circumstanceunder which the resin is used, as long as the strongly acidiccation-exchange resins have the same resin matrix and exchange groups,it becomes possible. to implement precise evaluation of the performanceof various resins. In particular, if the aforementioned method forevaluating the resin performance by using the sum of values, eachcalculated by multiplying an amount of PSS eluted in each molecularweight range by a corresponding weighting factor, is employed,independently of the structure of the resin matrix and the circumstanceunder which the resin is used, it becomes possible to determine, forexample, the deterioration degree or the deterioration tendency of theresin at a high accuracy and at a condition extremely easy to bedetermined, thereby achieving precise evaluation of the resinperformance.

Further, a method for controlling a water treatment system according tothe present invention comprises the steps of applying theabove-described method for evaluating a cation-exchange resin to anevaluation of the performance of a cation-exchange resin used in a watertreatment system; and determining a timing for replacing of thecation-exchange resin based on the result of the evaluation.

Especially, a preferable embodiment of the method for controlling awater treatment system according to the present invention is a methodfor controlling a water treatment system using the aforementioned methodwherein a weighting factor for indicating a degree concerning theperformance of the cation-exchange resin is preset for said eachmolecular weight range of PSS eluted, and the performance of thecation-exchange resin is evaluated by using the sum of values, eachcalculated by multiplying an amount of PSS eluted in said each molecularweight range by a corresponding weighting factor, as an index indicatingthe performance of the cation-exchange resin, and determining a timingfor replacing the cation-exchange resin. Particularly, a method forcontrolling a water treatment system is preferred wherein, using theabove-described method, an upper limit is set to the sum of values, eachcalculated by multiplying an amount of PSS eluted in said each molecularweight range by a corresponding weighting factor, and thecation-exchange resin is used in a range of the upper limit or less. Inthis control method, a method may be employed wherein, with respect tothe above-described sum, a criterion value, which is lower than theupper limit, is set for starting to prepare the replacement of thecation-exchange resin being used. By setting such an upper limit or acriterion value, use under the upper limit, which does not run a risk ofmalfunction of the water treatment system, becomes possible, and itbecomes possible to start a preparation for replacing the resin from thetime when the criterion value has been reached, and while continuing astable operation during a term required for the preparation, to performthe replacement of the resin at a timing before reaching the upper limitor at a timing having reached the upper limit. Namely, thecation-exchange resin can be used as long as possible within a termcausing no problem in performance, and after the use, the resin can bereplaced at an optimum timing.

Such a method for controlling a water treatment system according to thepresent invention is also suitable to be used for evaluating theperformance of a cation-exchange resin used in a condensatedemineralizer of a power plant and determining a timing for replacementof the cation-exchange resin based on the result of the evaluation.

In such method for evaluating a cation-exchange resin and method forcontrolling a water treatment system using the same according to thepresent invention, it becomes possible to evaluate a deteriorationdegree of a resin precisely by a single determination methodindependently of the structure of the resin matrix and the circumstanceunder which it is used. By employing this evaluation method, it becomespossible to stably continue a desirable operation and use acation-exchange resin as effectively as possible within a possible termfor use, the replacement cycle of the resin may be extended and the costrequired for the water treatment operation may be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between a converted valueaccording to the present invention and MTC (mass transfer coefficient)in examples of the present invention.

FIG. 2 is a graph showing a relationship between a sum of elutionamounts of PSS (polystyrenesulfonic acid) and MTC (mass transfercoefficient) in comparative examples using the same samples as thoseshown in FIG. 1.

FIG. 3 is a graph showing a relationship between a term for use (year)and a converted value according to the present invention in examples ofthe present invention.

FIG. 4 is a graph showing a relationship between a term for use (year)and a sum of elution amounts of PSS in comparative examples using thesame samples as those shown in FIG. 3.

THE BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be explained based on examples.

First, a plurality of molecular weight ranges were set in a molecularweight distribution of PSS eluted from a cation-exchange resin, and whenthe performance of a strongly acidic cation-exchange resin is evaluatedbased on a relationship of each molecular weight range with an amount ofPSS eluted in each molecular weight range, a weighting factor forindicating a degree concerning the performance of the strongly acidiccation-exchange resin was set for each molecular weight range asfollows.

Table 1 (Tables 1-1 and 1-2) shows a reduction degree of MTC (masstransfer coefficient) for each molecular weight range of a stronglybasic anion-exchange resin on which a standard substance of PSS whosemolecular weight is known was adsorbed by 100 mg/L relative to one literof the strongly basic anion-exchange resin (a relationship between amolecular weight distribution of PSS and MTC of the anion-exchangeresin). Where, although the MTC of a new strongly basic anion-exchangeresin is about 2×10⁻⁴ m/s, the reciprocal of a reduction ratio of eachMTC using an MTC relative to PSS with a molecular weight of 10,000 as abase of 1 was calculated, and with respect to a plurality of molecularweight ranges divided into fractions of a molecular weight of 10,000 ormore to less than 40,000, a molecular weight of 40,000 or more to lessthan 150,000, a molecular weight of 150,000 or more to less than1,000,000, and a molecular weight of 1,000,000 or more, theabove-mentioned reciprocal for the median value of each range or a valueclose thereto was set as a weighting factor (coefficient) for eachmolecular weight range. Using these weighting factors, the performanceof various strongly acidic cation-exchange resins used practically incondensate demineralizers, was evaluated by the method according to thepresent invention. The statuses of the evaluated sample resins are shownin Table 2 (classification and used years of the sample resins). SamplesA to G are all strongly acidic cation-exchange resins which had beenvariously used in condensate demineralizers of PWR-type nuclear powerplants. Among these samples, sample E was incidentally obtained as asample which had been excessively deteriorated due to usage for anextended period of time. TABLE 1-1 Relationship between PSS molecularweight distribution and MTC of anion-exchange resin Molecular weight6,500 10,000 40,000 150,000 1,000,000 MTC(×10⁻⁴ m/s) 2 1.7 1.2 0.9 0.2Reciprocal※ 0.85 1 1.4 1.9 8.5MTC of anion-exchange resin when PSS of each molecular weight isadsorbed by 100 mg per one liter of anion-exchange resin※Reciprocal of each MTC ratio on the basis of 1 for a molecular weightof 10,000

TABLE 1-2 Molecular weight 10,000˜40,000 40,000˜150,000150,000˜1,000,000 1,000,000 or more Coefficient 1.2 1.65 5.2 15(weighting factor)

TABLE 2 Classification of sample resins and Term for use Sample A B C DE F G Classification gel-type gel-type MR-type MR-type MR-typemicroporous- microporous- type type Term for use 0.4 0.7 2.7 1.2 4.2 1.20.5 (year) MTC(×10⁻⁴ m/s) 2 1.95 1.3 1.75 0.8 1.8 1.9All are strongly acidic cation-exchange resins.

As to the above-described samples A to G, the performance of therespective strongly acidic cation-exchange resins was determined.Namely, 50 mL of each strongly acidic cation-exchange resin was dippedin an aqueous solution of cupric sulfate (CuSO₄), 10 g of Cu wasadsorbed per one liter of the resin, the resin bearing Cu ions wasdipped in a hydrazine aqueous solution for 16 hours under a conditionwhere 1.5 equivalent of hydrazine relative to one liter of resinexisted, to deteriorate the resin acceleratedly. Then, the resin wasregenerated by circulating 5% hydrochloric acid aqueous solution throughthe resin at a rate of 400 g (as a 35% HCL aqueous solution) per oneliter of the resin, and the copper ions adsorbed on the resin weredesorbed from the resin. This desorption of copper ions followed byelution with eluant hydrazine aqueous solution (see below) prevented theresin from further deteriorating, thereby permitting exactly todetermine the accelerated deterioration. After the regenerated stronglyacidic cation-exchange resin deprived of copper ions was washed withdeionized water, the resin was dipped in 100 mL of an aqueous eluantsolution containing ammonia with a concentration of 1% and hydrazinewith a concentration of 0.2%, and heated at 40° C. and shaken for 16hours. After the shaking for 16 hours, the amount of PSS eluted in theaqueous eluant solution was determined by GFC (gel filtrationchromatography).

The data obtained for the respective PSS molecular weight ranges weredivided into the respective fractions of a molecular weight of 10,000 ormore to less than 40,000, a molecular weight of 40,000 or more to lessthan 150,000, a molecular weight of 150,000 or more to less than1,000,000, and in the respective fractions, amounts of PSS eluted andthe sum thereof were calculated. The results are shown in Table 3(amount of PSS eluted from sample strongly acidic cation-exchange resin[amount eluted in each molecular weight range and the sum thereof]).Further, the values calculated by multiplying the respective elutionamounts of the respective fractions of molecular weight (for therespective molecular weight ranges) by the respective weighting factorsshown in Table 1-2 of the respective molecular weight ranges and the sumthereof are shown in Table 4 (amount 2 of PSS eluted from samplestrongly acidic cation-exchange resin). TABLE 3 Amount of PSS elutedfrom sample strongly acidic cation-exchange resin Molecular weightCation- 150,000 or more 40,000 or more 10,000 or more Sum of exchangeresin 1,000,000 or more; less than 1,000,000 less than 150,000 less than40,000 10,000 or more A 0.30 0.16 0.45 0.28 1.19 B 0.63 0.33 1.09 1.083.13 C 3.65 0.48 1.72 0.98 6.83 D 1.44 0.27 0.98 0.89 3.58 E 6.27 1.514.02 4.63 16.4 F 0.29 0.35 3.74 6.89 11.3 G 0.10 0.31 2.75 4.80 7.96(Unit: mg PSS/L-strongly acidic cation-exchange resin)

TABLE 4 Amount of PSS eluted from sample strongly acidic cation-exchangeresin (x weighting factor) Molecular weight 150,000 or more 40,000 ormore 10,000 or more Sum of 1,000,000 or more less than 1,000,000 lessthan 150,000 less than 40,000 10,000 or more Coefficient (weightingfactor) 15 5.2 1.65 1.2 — Sample A 4.50 0.83 0.74 0.34 6.41 Sample B9.45 1.72 1.80 1.30 14.3 Sample C 54.8 2.50 2.84 1.18 61.3 Sample D 21.61.40 1.62 1.07 25.7 Sample E 94.1 7.85 6.63 5.56 114 Sample F 4.35 1.826.17 8.27 20.6 Sample G 1.50 1.61 4.54 5.76 13.4

FIG. 1 shows the result plotting a relationship between the sum of thevalues calculated by PSS elution amount×weighting factor (convertedvalues) and MTC (refer to Table 2) of strongly basic anion-exchangeresins used together with the respective samples A to G of stronglyacidic cation-exchange resins (relationship 1 between PSS elution amountand MTC). As comparison, FIG. 2 shows a relationship between the sum ofelution amounts themselves of PSS having a molecular weight of 10,000 ormore as listed in Table 3 and MTC (relationship 2 between PSS elutionamount and MTC). In FIG. 1, as is evident from the comparison with FIG.2, a correlation of the converted value with MTC appears destinct, andit is understood that, although it is difficult to precisely evaluatethe deterioration degree of the strongly acidic cation-exchange resinonly by observing the sum of PSS elution amounts, the deteriorationdegree can be determined and evaluated properly and very accurately byusing the sum of the values using weighting factors for the respectivemolecular weight ranges according to the present invention. Inparticular, it is understood that the correlationship shown in FIG. 1can be determined as an almost linear property independently of thestructure of a resin matrix and the circumstance under which it is used(for example, term of years used), and the deterioration degree can bedetermined easily and accurately, and further, stably.

Further, FIG. 3 shows a relationship between a term for use (year) ofthe strongly acidic cation-exchange resin and the sum of the valuescalculated by PSS elution amount×weighting factor (converted values).FIG. 4 shows relationships between terms for use (year) of the samesample strongly acidic cation-exchange resins and the sums of elutionamounts themselves of PSS having a molecular weight of 10,000 or more.Although samples F and G among these sample resins were resins which hadbeen used in an actual plant without any problem and it is clear thatthe sample resins F and G which had not been deteriorated appreciably asviewed from the values of MTC of strongly basic anion-exchange resinsused together with these strongly acidic cation-exchange resins as apair (shown in Table 2), as shown in FIG. 4, in the conventionalevaluation method based on the elution amount of PSS having a molecularweight of 10,000 or more, it is judged that the PSS elution amount isover a criterion value (usually, if the elution amount is more than 5mg/L-R, it is determined that a deterioration tendency is apparent.) anda deterioration tendency has become apparent, and therefore, arelationship between such a judgement does not reflect the actualsituation. In contrast, by using the sum of values using a weightingfactor for each molecular weight range in the evaluation methodaccording to the present invention, as is evident from FIG. 3, bothsamples F and G exhibit almost the same level of the sum as that in theother samples A, B and D which do not exhibit a deterioration tendency,and therefore, the performance of these sample resins F and G areregarded as good as those of the other sample resins A, B and D andthese evaluation results do reflect actual operating status of theseresins.

Therefore, by using the evaluation method according to the presentinvention, the deterioration degree of a strongly acidic cation-exchangeresin can be evaluated precisely.

Further, particularly in FIG. 3, it is understood that, from the data ofsample E which was obtained incidentally, an upper limit of theabove-described sum of the values calculated by PSS elutionamount×weighting factor may be set, for example, at about 100. As longas a strongly acidic cation-exchange resin is used under a condition ofthis upper limit 100 or less, it becomes possible to always use theresin within a range in which a deterioration degree does not exceed theupper limit. That is to say, replacing the resin if the upper limit hasbeen reached or approached permits to use a strongly acidiccation-exchange resin always stably and within a safe range.

However, merely for the control so that a deterioration degree does notexceed a loosely predetermined upper limit, it is possible to achievethe control by replacing the resin always at an early stage without runa risk of malfunction, but, if done so, there is a possibility that theresin is replaced in spite of existence of a sufficient remainingpossible term for use of the resin, and such replacement is notefficient from the viewpoints of operation costs. Accordingly, in a caseindicating such a property as shown in FIG. 3, if a value lower than theupper limit 100, for example, about 80, is set at a criterion value forstarting to prepare the replacement of the cation-exchange resin, from atime having reached this criterion value, in consideration of a termuntil a time approaching the upper limit, the replacement of the resincan be prepared with an enough time. This criterion value may beappropriately decided in accordance with the time required for thepreparation of the resin to be replaced. Thus, by setting a criterionvalue for starting to prepare the replacement of a cation-exchangeresin, it becomes possible to use the resin efficiently as much aspossible always in the usable range of the cation-exchange resin withoutexcess of the range, and extension of the resin exchange cycle and greatreduction of the running cost can be easily achieved without anyproblem.

Although the present invention is suitable to be adopted for theevaluation of a strongly acidic cation-exchange resin used in acondensate demineralizer of a power plant, as long as a preciseevaluation of a cation-exchange resin is required, the present inventioncan be applied to the control of any water treatment system.

Thus, in the method for evaluating a cation-exchange resin according tothe present invention, it becomes possible to evaluate a deteriorationdegree of the resin precisely by a specified single determinationmethod, independently of the structure of the resin matrix and thecircumstance under which it is used. By applying this evaluation methodto the evaluation of a strongly acidic cation-exchange resin used in acondensate demineralizer of a power plant, it becomes possible to stablycontinue a desirable operation of the condensate demineralizer and usethe cation-exchange resin as effectively as possible within a possibleterm for use, and the replacement cycle of the resin may be maximizedand the cost required for the operation may be reduced.

INDUSTRIAL APPLICATIONS OF THE INVENTION

The method for evaluating a cation-exchange resin according to thepresent invention can be applied to any field requiring a preciseevaluation of a cation-exchange resin, and in particular, it is suitableto be applied to the evaluation of a strongly acidic cation-exchangeresin used in a condensate demineralizer of a power plant. The methodfor controlling a water treatment system using this evaluation methodaccording to the present invention can be applied to any field requiringa precise evaluation of a cation-exchange resin and a determination ofan optimum resin replacement time and in particular, it is suitable tobe applied to the evaluation of a strongly acidic cation-exchange resinused in a condensate demineralizer of a power plant and the decision ofthe resin replacement timing.

1. A method for evaluating a acidic cation-exchange resin wherein saidstrongly acidic cation-exchange resin is contacted with an aqueouseluting solution and polystyrenesulfonic acid being eluted from saidresin is measured, comprising the steps of: setting a plurality ofmolecular weight ranges in a molecular weight distribution of saidpolystyrenesulfonic acid eluted; and evaluating a performance of saidstrongly acidic cation-exchange resin based on a relationship of eachmolecular weight range with an amount of said polystyrenesulfonic acideluted in said each molecular weight range.
 2. The method for evaluatinga cation-exchange resin according to claim 1, wherein a weighting factorfor indicating a deterioration degree of said performance of saidstrongly acidic cation-exchange resin is preset for said each molecularweight range, and said performance of said strongly acidiccation-exchange resin is evaluated by using the sum of values, eachcalculated by multiplying an amount of said polystyrenesulfonic acideluted in said each molecular weight range by a corresponding weightingfactor, as an index indicating said performance of said strongly acidiccation-exchange resin.
 3. The method for evaluating a cation-exchangeresin according to claim 2, wherein said weighting factor is preset forsaid each molecular weight range, based on a variation degree of aproperty of an anion-exchange resin ascribed to a polystyrenesulfonicacid eluted from said strongly acidic cation-exchange resin when saidstrongly acidic cation-exchange resin is used in a form of a mixed bedwith said anion-exchange resin.
 4. The method for evaluating acation-exchange resin according to claim 3, wherein said weightingfactor for said each molecular weight range is set based on a variationdegree of a property of an anion-exchange resin exhibited when arepresentative molecular weight is set for said each molecular weightrange and a standard polystyrenesulfonic acid having said representativemolecular weight as a known molecular weight is adsorbed on saidanion-exchange resin.
 5. The method for evaluating a cation-exchangeresin according to claim 1, wherein said plurality of molecular weightranges are set in a range of molecular weight of 10,000 or more.
 6. Themethod for evaluating a cation-exchange resin according to claim 1,wherein a copper ion and/or an iron ion are adsorbed on said stronglyacidic cation-exchange resin, a hydrazine aqueous solution is contactedtherewith to deteriorate said resin acceleratedly, and after said copperion and/or said iron ion are desorbed, said aqueous eluting solution iscontacted to elute said polystyrenesulfonic acid into said aqueouseluting solution.
 7. The method for evaluating a cation-exchange resinaccording to claim 1, wherein an aqueous solution containing an ammoniaand a hydrazine is used as said aqueous eluting solution.
 8. The methodfor evaluating a cation-exchange resin according to claim 1, wherein aperformance of a cation-exchange resin used in a condensatedemineralizer of a power plant is evaluated.
 9. A method for controllinga water treatment system comprising the steps of: applying a method forevaluating a cation-exchange resin, wherein a strongly acidiccation-exchange resin is contacted with an aqueous eluting solution, andwhen polystyrenesulfonic acid being eluted from said resin is measured,a plurality of molecular weight ranges are set in a molecular weightdistribution of said polystyrenesulfonic acid eluted, and a performanceof said strongly acidic cation-exchange resin is evaluated based on arelationship of each molecular weight range with an amount eluted insaid each molecular weight range, to an evaluation of a cation-exchangeresin used in a water treatment system; and determining a timing forreplacement of said cation-exchange resin based on the result of saidevaluation.
 10. The method for controlling a water treatment systemaccording to claim 9, wherein a weighting factor for indicating adeterioration degree of said performance of said cation-exchange resinis preset for said each molecular weight range of said cation-exchangeresin used in said water treatment system, and said performance of saidcation-exchange resin is evaluated by using the sum of values, eachcalculated by multiplying an amount eluted in said each molecular weightrange by a corresponding weighting factor, as an index indicating saidperformance capability of said cation-exchange resin.
 11. The method forcontrolling a water treatment system according to claim 10, wherein anupper limit is set to said sum of values, each calculated by multiplyingan amount eluted in said each molecular weight range by a correspondingweighting factor, and said cation-exchange resin is used in a range ofsaid upper limit or less.
 12. The method for controlling a watertreatment system according to claim 11, wherein with respect to saidsum, a criterion value, which is lower than said upper limit, is set forstarting to prepare the replacement of said cation-exchange resin beingused.
 13. The method for controlling a water treatment system accordingto claim 9, wherein a performance of a cation-exchange resin used in acondensate demineralizer of a power plant is evaluated, and based on theresult of the evaluation, a timing for replacement of saidcation-exchange resin is determined.